1. Field of Invention
The invention relates to a device for calibrating membrane tubes which are extrudable by an extrusion die, as well as a method for calibrating such membrane tubes.
2. Description of the Prior Art
Devices for calibrating membrane tubes have long been disclosed. In addition to being used for calibrating the membrane tube, said devices are also used for quickly cooling the membrane tube. In the process, the cooling speed has a considerable impact on the transparency of the future membrane. Generally, a high cooling speed also means high transparency. For this reason, a fluid is used for the cooling, because fluids have a high thermal capacity compared to a gas. Water is often used for the sake of easy handling. However, because the gravitational force has a considerable impact on a fluid compared to a gas, the membrane tubes cooled in this fashion are extruded from an extrusion die toward the direction of the gravitational force, i.e. downward.
This kind of device is described in the disclosure statement DE 2 005 182. The device disclosed there comprises a ring along the interior wall of which the extruded membrane tube can be guided. This means that the interior wall of the ring delimits the space for the membrane tube, so that it is defined by the ring's diameter and hence always has the same diameter as the ring. The ring is often referred to as “calibration sleeve” in technical circles.
Furthermore, the device disclosed there comprises a cooling liquid provision appliance with which a cooling liquid can be provided which can be conveyed to between the interior wall and the membrane tube. The cooling liquid is conveyed through the porous wall of the ring and reaches the space between the exterior surface of the membrane tube and the interior wall of the ring, so that the two are not touching each other, but a good heat exchange is possible. Transparent membranes can be produced in this manner.
However, it is a disadvantage that only membrane tubes made of certain materials can be produced with these kinds of devices. With certain materials, such as for example polypropylene (PP), the just extruded area of the membrane tube located above the frost line is unable to retain the already cooled part of the membrane tube located further down. The so-called inherent retention force is close to zero in some materials. Consequently, these kinds of materials cannot be produced by means of a described device. In order to manufacture transparent membranes made of these kinds of materials, other devices and methods, including for example the production of so-called flat membranes, need to be used. But these kinds of membranes cannot be manufactured efficiently this way.